Mounting arrangement for high-frequency electro-optical components

ABSTRACT

An auxiliary component such as a bias inductance ( 6 ) associated with a laser source ( 2 ) is mounted generally “upright”, that is with its major dimension substantially orthogonal to the general plane of the submount (S) supporting both the laser ( 2 ) and the auxiliary component ( 6 ). The inductor ( 6 ) is preferably mounted at a location displaced laterally with respect to the lasing direction (X) of the laser source. The arrangement preferably includes a submount (S) with a recess ( 13 ) and at least part of the laser driver is arranged in the recess so that the driver ( 3 ) has an end surface extending from the recess substantially flush with the pad ( 12 ) for mounting the laser source. The arrangement minimises surface occupation as well as RF and EMI parasitic effects related to wirebonding.

[0001] The present invention relates to mounting arrangements forhigh-frequency electro-optical components. Exemplary of such componentsare semiconductor laser sources for optical communications and thebias/driver circuitry currently associated with such sources.

[0002] A standard mounting arrangement adopted for electronic componentssuch as integrated circuits (IC's) includes a flat base member or board(submount) having electrical connections provided on at least one of itssurfaces in the form of metallic areas such as metallic strips or pads.

[0003] Various techniques may be resorted to in order to connect thecomponents to such strips/pads. These techniques may include providingthe components with leads or pins for soldering to the conductivestrips/pads or more sophisticated solutions such as “flip-chip” bonding,SMD technology, possibly including the use of electrically conductiveglues, and the like.

[0004] Such prior art solutions are essentially based on atwo-dimensional or bi-dimensional layout, where the lengths and widthsof the components limit the number of components for surface unit. Thismay lead to fairly penalising situations in terms of submount areaoccupied, even if small size SMD components and compact layouts andassemblies are used.

[0005] Also, optimisation of RF performance is severely hampered as thelayout may be influenced by e.g. passive components used for biasnetworks.

[0006] In operation at very high frequencies (in the range of 10 Gbit/sand above) in connection with opto-electronic components such assemiconductor laser sources, the need is strongly felt of reducinginasmuch as possible the wirebonding length, while also simplifying thegeometry of connections provided at the submount level.

[0007] Additional requirements arise as a result of the presence of theopto-electrical components. For instance, semiconductor lasers have amain or front facet from which a laser beam is generated that is to beinjected into an optical waveguide along a substantially unimpededpropagation path. Such semiconductor lasers also currently have a backfacet from which a secondary laser beam is produced. The secondary laserbeam may be collected and detected by an opto-electrical component, suchas a photodiode, e.g. for control purposes. Undesired back reflection ofsuch secondary laser beam towards the laser source must be avoided asthis may interfere with proper laser operation.

[0008] An environment where the problems outlined in the foregoing areparticularly evident is an integrated optical transmitter, including alaser diode and a laser driver as the basic components.

[0009] In the presence of a laser capacitance of 1 or 2 pF and acapacitance of the output pads of the laser driver of a few pF, aparasitic inductance due to the connections between the laser diode andthe laser driver of, say, 1 nH may generate a resonance frequency withinthe signal band, thus producing distortion and a penalty in terms of thesystem bit error probability (BER).

[0010] A very small clearance must be kept between the pad for mountingthe laser diode and the laser driver, so that the correspondingwirebonding is as short as possible, preferably less than 1 mm.Similarly, the ground connections of the laser driver include parasiticinductances that may affect the behaviour of the assembled circuit andmust be kept as low as possible.

[0011] Also, the circuit must include a laser bias network generallycomprised of at least one SMD inductor. Such an inductor isintrinsically a rather “obtrusive” component in comparison with thelaser source and the other auxiliary components located in the vicinitythereof. The inductor may be e.g. a SMD component in the form of aparallelepiped about 1-1.5 mm i.e. 1,500 micron long and roughly squareend faces having sides about 500-750 micron long. This contrasts withthe laser dimensions being typically in the range of 300×200 micron basearea with a thickness of e.g. 90 microns.

[0012] In order to be truly effective, the bias inductance has to bepositioned in the close vicinity of both the laser diode and the laserdriver, which strongly penalises the designer's freedom in devising thecircuit geometry.

[0013] The object of the present invention is thus to provide a solutionmeeting in a thoroughly satisfactory manner the conflicting needsconsidered in the foregoing.

[0014] According to the present invention, that object is achieved bymeans of an arrangement having the features set forth in the annexedclaims.

[0015] Essentially, the invention is based on the concept of exploitingalso the “third” dimension of the component (e.g. the bias inductor)involved in order to be able to assemble a higher number of componentsover the same area. The third dimension in question is the height, thatis the dimension of the inductor in the direction orthogonal to thegeneral plane of the submount. As this is quite often the majordimension of the inductor, a more efficient use of space in terms ofmechanical layout may be achieved, thus giving rise to smallerassemblies while making the distances between the components smaller.

[0016] Also, the inductor is preferably mounted at a location displacedlaterally with respect to the lasing direction of the laser source, thatis the direction of alignment of the front and back (lasing) facets ofthe laser. Such an arrangement of the inductor simultaneously ensuresthat the inductor does not impede the propagation path of the main laserbeam generated by the laser source, while permitting the possiblearrangement of a photodetector to collect the secondary radiation fromthe back facet of the laser source.

[0017] The whole of the foregoing also permits optimisation ofwirebonding design especially in respect of RF performance related toreducing the very critical wirebonding length between the laser and theRF laser driver output. This in turn reduces the influence of a radiofrequency (RF) and electromagnetic interference (EMI) phenomena, thuspermitting laser operation at higher speeds.

[0018] In a particularly preferred embodiment, the arrangement ofinvention includes a submount with a recess and at least part of thelaser driver is arranged in the recess so that the driver has an endsurface extending from the recess substantially flush with the pad formounting the laser source.

[0019] The invention will now be described, by way of example only, byreferring to the enclosed drawings, wherein:

[0020]FIG. 1 is a general plan view of a submount for high-frequencycomponents incorporating the arrangement of the invention, and

[0021]FIG. 2 is a cross sectional view taken along line II-II of FIG. 1.

[0022] In FIG. 1 reference 1 indicates as a whole an assembly for ahybrid integration of an optical transmitter. This includes asemiconductor laser 2 having associated a number of ancillaryunits/components arranged onto a submount generally indicated S.

[0023] Semiconductor laser 2 has a main or front facet 2 a from which alaser beam is generated along an axis designated X to be injected (byknown means, e.g. a lens) into an optical waveguide such as an opticalfiber along a substantially unimpeded propagation path.

[0024] Semiconductor laser 2 also has a back facet 2 b from which asecondary laser beam is produced. The secondary laser beam may becollected and detected by an opto-electrical component, such as aphotodiode, e.g. for control purposes.

[0025] The fiber and the lens comprising the optical radiation featureof transmitter 1 as well as the control photodiode hinted at in theforegoing are of a type well known in the art. Correspondingly, thesewere not shown in the drawing in order to avoid rendering the drawingunnecessarily complex.

[0026] Assembly 1 further includes i.a. a laser driver circuit 3 thatprovides laser 2 with a driver signal over a signal path. This isusually comprised of two bonding wires 4 in order to minimise the pathinductance.

[0027] Reference 5 denotes a dc feed pad that, in co-operation with abias inductance 6, defines a circuit for setting the required bias levelfor laser 2. Such a level is usually rendered adjustable by the laserdriver unit 3 via a bias line 7.

[0028] Other sets of bonding lines collectively designated as 8 and 9connect the laser driver circuit 3 with ground pads designated 10 and11, respectively. These bonding lines are used for feeding the laserdriver while they in fact belong to the return path for the RF signaltowards ground.

[0029] Pads 5, 10 and 11 are preferably provided in the form ofmetallised (e.g. gold) areas over the submount S supporting the wholeassembly, a substantially similar metallised area or pad 12 housing bothsemiconductor laser 2 and inductor 6.

[0030] Pad 12 thus provides electrical connection for the RF (e.g. 10Gbit/s) signal path towards laser 2 via bonding wires 4 as well as forthe d.c. bias path for laser 2 from pad 5 via inductor 6.

[0031] Submount S is typically of a ceramic material such as Al₂O₃ andpreferably includes a recessed or “sunken” area 13 for mounting at leastpart of driver 3. Driver 3 may thus be arranged in the recess 13 so thatdriver 3 has its upper surface extending flush with the pad 12 formounting laser source 2 and the other elements of the assembly. Thisenables the corresponding wirebonding lengths to be minimised.

[0032] As better appreciated in the cross-sectional view of FIG. 2,instead of being mounted “flat” onto pad 12, bias inductance 6 ismounted “upright”, that is with its major dimension (i.e. its longerdirection of extension) arranged substantially orthogonal orperpendicular to the general plane of surface S′ of submount S.

[0033] This arrangement, that is inductance 6 being mounted with itslongest dimension substantially orthogonal to the plane of surface S′ ofsubmount S, causes inductance 6 to have a much smaller footprint onmetallised pad 12 than in the case of a flat mounting. This enableslocating inductance 6 very close to laser 2 with a length reduction inthe very critical wirebonding lines 4, 8, 9 of the RF path.

[0034] Inductor 6 is usually comprised of a SMD component in the form ofa parallelepiped about 1-1.5 mm i.e. 1,500 micron long and roughlysquare end faces having sides about 500-750 micron long. Inductor 6 maythus be preferably mounted onto metallised pad 12 by resorting to aconductive glue layer 15.

[0035] As better appreciated in the plan view of FIG. 1, inductor 6 ispreferably mounted at a location displaced laterally with respect to thelasing direction X of laser source 2, that is the direction of alignmentof the front and back (lasing) facets 2 a, 2 b of the laser.

[0036] As a consequence, inductor 6 will not impede the propagation pathof the main laser beam generated from the front facet 2 a of lasersource 2 towards the optical waveguide (not shown) into which the laserbeam is injected. Possible arrangement of a photodetector to collect thesecondary radiation from the back facet 2 b of the laser source is alsopermitted while avoiding that this may result in an undesired extensionof the wirebonding towards laser 2.

[0037] As used herein, inductor 6 being “displaced laterally,” withrespect to the lasing direction X of laser source 2 also encompasses analternative mounting arrangement (not shown) where inductor 6 is atleast partly aligned—i.e. not located as a whole side-off—with respectto lasing direction X. In such a case inductor 6 may have a face orsurface possibly exposed to and likely to be impinged upon by thesecondary radiation from back facet 2 b of laser 2. In that case,inductor 6 is “displaced laterally” with respect to the lasing directionX by simply arranging it in such a way that such face or surface istilted laterally with respect to lasing direction X so that anyradiation from the laser back facet along direction X is reflected awayfrom such direction and is not returned back towards the laser source.

[0038] Of course, without prejudice to the underlying principle of theinvention, the details and embodiments may vary with respect to what hasbeen described and shown by way of example only, without departing fromthe scope of the present invention as defined in the annexed claims.

1. A mounting arrangement for a laser source (2) and at least oneauxiliary component (6) associated therewith, said at least oneauxiliary component having a major dimension, said laser source (2) andsaid auxiliary component (6) being mounted on a submount (S) having ageneral plane of extension, characterised in that said at least oneauxiliary component (6) is mounted with said major dimensionsubstantially orthogonal to said general plane of said submount (S). 2.The mounting arrangement of claim 1, characterised in that saidauxiliary component is a bias inductance (6) for said laser source (2).3. The mounting arrangement of either of claims 1 or 2, characterised inthat said laser source has a lasing direction (X) and said auxiliarycomponent is displaced laterally with respect to said lasing direction(X) of said laser source (2).
 4. The mounting arrangement of claim 3,characterised in that said laser source (2) includes front (2 a) andback (2 b) lasing facets aligned along said lasing direction (X), saidauxiliary component (6) has a surface exposed to radiation from saidback facet (2 b) of the laser source (2), and in that said surface istilted laterally with respect to said lasing direction (X) so thatradiation from said laser back facet (2 b) along said lasing direction(X) is reflected away from such direction.
 5. The mounting arrangementof any of the previous claims, characterised in that said at least oneauxiliary component (6) is mounted onto said submount (3) by means ofconductive glue (15).
 6. The mounting arrangement of any of the previousclaims, characterised in that said at least one auxiliary component (6)is in the form of an SMD component.
 7. The mounting arrangement of anyof the previous claims, characterised in that it includes anelectrically conductive area or pad (12) onto which both said lasersource (2) and said auxiliary component (6) are mounted.
 8. The mountingarrangement of any of the previous claims, characterised in that itincludes an electrically conductive area or pad (12) for mounting saidlaser source (2) as well as a driver (3) for said laser source (2), andin that said submount (S) has an outer surface (S′) and a recess (13)recessed with respect to said outer surface (S′), wherein at least partof said laser driver (3) is arranged in said recess (13) so that thedriver has an end surface extending from the recess substantially flushwith said conductive pad (12) for mounting said laser source (2).